Under the Code Division Multiple Access (CDMA) mobile communication mode, conventionally, transmission and reception are performed under a lowered diffusion rate, as a method for executing data communication at a high speed. This leads to lowered diffusion gain which often incurs signal degradation, and hence the data channel requires a high power.
Also, the HSDPA format, expected to be put to practical use in future, employs 16QAM modulation for achieving data communication at an even higher speed, which requires a higher channel power than in the current system which employs QPSK modulation.
Here, the HSDPA refers to a data transmission mode that employs the High Speed-Physical Downlink Shared Channel (hereinafter, HS-PDSCH), which offers a faster downstream transmission speed.
Under the HSDPA mode, data is not constantly transmitted, and reception power suddenly increases (+ΔP) when the data emerges as shown in a fluctuation example of downstream reception power of FIG. 10, when seen from the receiving terminal side.
Accordingly, the Auto Gain Control (hereinafter, AGC) is unable to follow up and the level of the received signal is saturated, and hence a receiving error is incurred. If such receiving error is always incurred upon starting up the HS-PDSCH transmission the throughput is degraded, and therefore some measures have to be taken.
In FIG. 10, the High Speed-Shared Control Channel (hereinafter, HS-SCCH) is a channel for transmitting control information therethrough to a mobile communication terminal. The Common Pilot Channel (hereinafter, CPICH) is a channel for a common pilot. The Primary Common Control Physical Channel (PCCPCH) is a first common control physical channel.
Meanwhile, a technique has been proposed that controls, in the HSDPA mode, a total sum of the power required by all the channels for transmission from a base station to a mobile communication terminal, to be constant. The technique is intended for preventing fluctuation of interference power with respect to another user, arising from the fluctuation of the downstream transmission power.
More specifically, the technique includes controlling the HS-PDSCH transmission power so as to maintain the total sum of the power for the CPICH, the Dedicated Physical Channel (hereinafter, DPCH), and the HS-PDSCH at a constant level (for example, refer to patented document 1).    [Patent document 1] JP-A No. 2002-261687